Lesson 1 of 11 · 7 min
Oxidation and reduction: the classical view
NCERT §7.1
A magnesium ribbon burns with a white flame and leaves a white ash. The metal has joined with oxygen, and that joining is what chemists first called oxidation.
The lesson in notes
In short
Originally, oxidation meant adding oxygen to a substance, as when magnesium burns to MgO or sulphur burns to SO₂. Oxygen is not the only reagent that counts.
The idea was broadened to cover removal of hydrogen: in 2H₂S + O₂ → 2S + 2H₂O, hydrogen is taken away from H₂S, so H₂S is oxidised.
It was widened again to adding an electronegative element: Mg + F₂ → MgF₂, Mg + Cl₂ → MgCl₂ and Mg + S → MgS all count as oxidation of magnesium.
Removal of an electropositive element is oxidation too: when K₄[Fe(CN)₆] reacts with H₂O₂ to give K₃[Fe(CN)₆], one potassium is removed per complex unit.
Reduction is the reverse in each case: removing oxygen (2HgO → 2Hg + O₂), adding hydrogen (ethene + H₂ → ethane), removing an electronegative element (2FeCl₃ + H₂ → 2FeCl₂ + 2HCl) or adding an electropositive one (2HgCl₂ + SnCl₂ → Hg₂Cl₂ + SnCl₄, where mercury(II) chloride is reduced).
Oxidation and reduction always happen together; one substance cannot be oxidised unless another is reduced. Such paired reactions are called redox reactions.